Designing a Logic Model for Mobile Maternal Health e-Voucher Programs in Low- and Middle-Income Countries: An Interpretive Review

Despite the increasing transition from paper vouchers to mobile e-vouchers for maternal health in low- and middle-income countries, few studies have reviewed key elements for program planning, implementation, and evaluation. To bridge this gap, this study conducted an interpretive review and developed a logic model for mobile maternal health e-voucher programs. Pubmed, EMBASE, and Cochrane databases were searched to retrieve relevant studies; 27 maternal health voucher programs from 84 studies were identified, and key elements for the logic model were retrieved and organized systematically. Some of the elements identified have the potential to be improved greatly by shifting to mobile e-vouchers, such as payment via mobile money or electronic claims processing and data entry for registration. The advantages of transitioning to mobile e-voucher identified from the logic model can be summarized as scalability, transparency, and flexibility. The present study contributes to the literature by providing insights into program planning, implementation, and evaluation for mobile maternal health e-voucher programs.


Introduction
Maternal mortality is known as one of the key indicators of health inequity due to its disproportionate occurrence among different socioeconomic groups [1]. According to the World Health Organization (WHO) and the Global Burden of Disease Study 2019, developing countries account for 99% of maternal mortality [1,2]. To minimize probabilities of obstetric complications and maternal deaths that are avoidable, at least four antenatal care (ANC) visits and four postnatal visits are recommended, but the coverage of basic maternal health services varies widely [3,4]. A previous study reported that overall antenatal care coverage in 2013 was 48.1% for developing countries compared to 84.8% for developed countries [5].
These dramatic gaps are expected to persist or may even get worse due to the prolonged COVID-19 pandemic. Specifically, the perceived barriers and the actual limitations in access during the COVID-19 pandemic can result in a low utilization of maternal health services. In Ethiopia, for example, a study involving 389 pregnant women reported 55.5% late or missed ANC visits during the COVID-19 pandemic period and, of those, 56% were due to fear of COVID-19 infection and 33% were due to interrupted maternal services [6]. According to COVID-19 surveys from India and DR Congo, about 50% of women had difficulties in accessing health facility during COVID-19 restrictions [7].
More importantly, the unmet maternal health needs in low-resource settings have been largely attributed to lack of financial resources. For example, the Performance Monitoring for Action survey from Kenya found that 91% of the female respondents lost household income partially or completely since COVID-19 restrictions, which could lead to the worst case scenario in terms of maternal health care service utilization [7]. In fact, financial constraints have been considered as one of the main causes for insufficient supply and demand for maternal health services in the past years [8]. A systematic review on access barriers to obstetric care in sub-Saharan Africa identified that the principal barrier was limited household resources or income [9]. Recent reports from the Demographic and Health Survey conducted during 2019 and 2020 in Liberia, Gambia, or Sierra Leone found that the most frequently mentioned barrier for women to get access to health services was getting money for treatment [10][11][12]. Another empirical study on access to healthcare among 307,611 sub-Saharan African women also pinpointed that the predominant barrier was money [13].
To tackle these maternal health disparities, huge financial contributions have been made over the past decades in the form of foreign aid or development assistance. Particularly, the maternal health issue has been given much attention as one of the health focus areas for development assistance since the Millennium Development Goals (MDGs) era. Furthermore, a previous study provided empirical evidence for the growing development assistance for maternal and child health since 2010, compared to other health focus areas such as HIV/AIDS, which showed a flattened or downward curve [14]. Given the fact that cost-effective maternal health services have been unequally available in many parts of the world, donors have used a number of innovative approaches to deliver these services for the underserved populations. One of the interventions to overcome financial barriers is results-based financing. Results-based financing is an umbrella term for a mechanism that provides financial incentives to the provider or service users only if pre-defined results are achieved [15].
There are two different approaches to results-based financing-supply-side and demand-side-depending on the focus of the scheme [16]. Supply-side financing offers incentives to providers, whereas demand-side financing provides subsidies for services or merit goods directly to the beneficiaries [17]. A voucher program is one of the methods for demand-side financing, which has been employed to remove barriers to accessing various health services, especially maternal and reproductive health. The rationale behind the maternal health voucher programs is that it can encourage care-seeking behavior by alleviating the burden of costs associated with maternity care services. Maternal health voucher programs have been implemented in low-and middle-income countries (LMICs) such as Kenya, Uganda, Pakistan, and Bangladesh. Evidence on the effectiveness of these programs has also been reported in previous studies, which mostly demonstrated the increase in maternal health service utilization after the implementation of voucher programs [18][19][20][21].
Recently, an electronic voucher or e-voucher involving mobile phone systems for maternal health has been experimented as an alternative to physical tokens or paper coupons. Mobile e-vouchers have the potential for improving the current operational process in various ways, such as real time tracking and monitoring or emergency communication. Transition from paper vouchers to mobile e-vouchers can allow transparency for reducing fraud and flexibility by adding or adjusting included services. A plastic card that contains bar codes is one option to deliver e-vouchers, but mobile app or SMS-based e-vouchers can be more feasible in LMICs where mobile subscription rates are getting higher. In addition, mobile money has been widely used in the daily lives for many sub-Saharan African countries such as Ghana, Kenya, or Tanzania. In Kenya, for example, a mobile maternal health e-voucher program integrated mobile money for providing maternal health services along with transportation cost subsidies [22].
As the mobile e-voucher system is a new approach with short history, little is known about the best practices or implementation strategies in the context of LMICs. For instance, a recent report on e-vouchers for family planning services addressed that no data were found in terms of comparative effectiveness between e-voucher and paper vouchers, or the impact of e-vouchers on health outcomes [23]. In this context, this study aimed to design a logic model for mobile maternal health e-voucher programs using an interpretive review approach. In the first step, evidence was synthesized from both the traditional and mobile e-voucher programs for maternal health in LMICs through an interpretive review approach. Based on this evidence, key components required to design a mobile e-voucher program for maternal health were synthesized and suggested within a logic model framework. For decades, logic models have been used for communicating a program cycle ranging from planning, implementation to monitoring, and evaluation [24]. In doing so, this study will provide recommendations for a successful transition from paper to mobile e-voucherd for maternal health services in LMICs.

Search Strategy and Review Process
To identify the studies on maternal health voucher programs in the context of LMICs, electronic databases (MEDLINE through Pubmed, EMBASE, and Cochrane Database of Systematic Reviews) were searched. The review question for this interpretive evidence synthesis is as follows: "What are the programmatic elements that should be considered to develop and evaluate a mobile maternal health e-voucher programs in LMICs?". After several trials for finalizing the optimal search terms, "maternal" and "voucher" were chosen to achieve the balance between the sensitivity and specificity of the search results [25]. In other words, only two representative terms for the population and intervention parts of the conventional PICO format for a systematic review were built into the search strategy. No restrictions were applied to the country or publication type, although studies on highincome countries were to be excluded during the screening process. Additional search was performed from the grey literature and references of the previous review studies. The search included articles and reports published up until May 2021.
The authors (S.L. and A.j.A.) independently reviewed the retrieved articles and reports based on the titles and abstracts. After removing duplicates and irrelevant studies from this first stage, a full-text review was conducted. The disagreement during the selection process was resolved by a discussion between the two authors.

Eligibility Criteria for Review
The following inclusion criteria were used for this interpretive review. First, the study population should be defined as women of all ages who were either pregnant or within 42 days of the conclusion of pregnancy for the postnatal period, as defined by the WHO [26]. Second, the study should deal with a voucher program for maternal health services in LMICs, as defined by the World Bank's classification by income level [27]. Third, the study should provide implications regarding inputs, activities, outputs, and outcomes of the voucher program. Fourth, the study should be published in English.
Studies were excluded for the following conditions. First, if the full-text was not available or there was only an abstract for conference presentation, the study was excluded. Second, studies that discussed voucher programs for goods or services that were not directly related to maternal health were excluded. Third, review articles were excluded, but the individual studies identified from the relevant reviews that met the inclusion criteria were included for the evidence synthesis.

Data Extraction and Synthesis
After reviewing the retrieved studies based on the inclusion and exclusion criteria, the information about the setting, target population, and included services under the voucher scheme were extracted. Specifically, key programmatic elements for the inputs, activities, outputs, and short-term and long-term outcomes were analyzed and transcribed into the logic model framework. As discussed in a previous study that used the systematic review approach for building a logic model, each element for the program planning, implementation, monitoring, and evaluation was categorized into the framework [28]. The logic model was developed under the assumption that the key elements identified from the traditional maternal health voucher programs, as well as mobile e-voucher programs, should be considered for a mobile maternal health e-voucher programs, because the funda-mental structure or cycle of the program would not be drastically changed by the transition from paper-based voucher to mobile e-vouchers [23]. The developed logic model was cross-validated with another logic model that was recently published as part of a protocol for a mobile maternal health e-voucher program in Cameroon [29]. This study is the only published protocol with a logic model so far.

Quality Appraisal
Conventional systematic review studies that aim for finding evidence of effectiveness or testing a theory usually assess the quality of the included studies either by (1) including only a certain study designs (e.g., randomized controlled trial) or (2) using structured quality assessment tools for a specific study design. However, these approaches are not feasible for the scope and purpose of our study, because the goal is to give insights into program design and evaluation by including various types of relevant publications with sufficient evidence, regardless of the study design. For example, the studies identified in this review include reports or qualitative studies that are generally excluded in conventional systematic reviews. Therefore, we employed suggestions by Dixon-Woods et al., which discussed a quality assessment method for maximizing the inclusion and contribution of diverse empirical studies [30]. This approach was originally recommended by the National Health Service of the United Kingdom and was adapted for an interpretive review. The five criteria for informing judgements about quality of studies are as follows [30].

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Are the aims and objectives of the research clearly stated? • Is the research design clearly specified and appropriate for the aims and objectives of the research? • Do the researchers provide a clear account of the process by which their findings we reproduced? • Do the researchers display data to support their interpretations and conclusions? • Is the method of analysis appropriate and adequately explicated?
Each of the above five appraisal questions were scored 1 if yes, and 0 if no. The maximum total score was 5 and the minimum was 0. However, no studies were excluded on the basis of the quality appraisal score.

Results
A total of 317 studies were identified and screened for the title and abstract. Of those, 208 were excluded and 109 articles were reviewed. Among 109 studies, 25 were excluded for the following reasons. First, three conference abstracts and three studies without fulltext were excluded. Second, eight review articles were not included, but their references were screened thoroughly to identify any eligible studies to be included in the analysis. Third, 11 studies not related to the maternal voucher program for pregnant women were excluded, such as family planning vouchers for women with HIV or urban youth [31,32]. Therefore, 84 studies were included for interpretive synthesis of the evidence. A flow diagram for the selection process, based on the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines, is illustrated in Figure 1.      The benefits package for the maternal health voucher programs can be divided into three components: maternal health services or products, transportation, and communication. First, maternal health services that were covered under the voucher scheme generally included four ANC visits, institutional delivery, and one PNC visit. For some of the programs in Pakistan, India, and Cambodia, three ANC visits instead of four were specified under the voucher scheme. In addition, maternal health-related products such as sanitary products for clean and safe delivery were provided under some voucher programs. Second, the transportation costs were covered either for emergency referral only or travel for any maternal health-related visits. The transportation support was provided as a stand-alone voucher program or in combination with maternal health services or products. Third, communication was one of the important components for maternal health voucher programs, in that it allowed for behavior change communication and emergency contacts. The communication component was enabled by the mobile phone system that was introduced as part of the mobile phone credit voucher program or mobile maternal health e-voucher programs.
Fourteen programs (51.9%) involved both maternal services and transportation. Five programs (18.5%) offered transportation alone, while four programs (14.8%) covered only maternal services or products. One program (3.7%) provided mobile phone credit vouchers for communication. Three programs (11.1%) that employed a mobile money platform or e-vouchers had benefits package with all three components-maternal services or products, transportation and communication.
As for the quality appraisal of the included studies, each study was scored based on the five appraisal questions suggested by Dixon-Woods et al. The quality appraisal scores ranged from 1 to 5, and the average score was 4.34 (SD 0.97) and 4.68 (SD 0.55) for Asia or the Middle East and sub-Saharan Africa, respectively.

Studies Involving Mobile Phone System
Out of 14 maternal health voucher programs in sub-Saharan Africa, seven (50%) programs involved a mobile phone system. Two programs in Uganda adopted the mobile phone system midway through the implementation [35,116], whereas three programs in Kenya were initially designed for using mobile money or a mobile e-voucher platform [75,76,78]. One study from Cameroon was discussed as a pilot protocol for the mobile e-voucher [29] and another program in Tanzania provided mobile phone credit vouchers for direct two-way communication between pregnant women and healthcare providers [79,80].
These programs can be categorized into four different types. First, programs in Uganda used the mobile phone system for operational process, such as reimbursement via mobile money for the drivers or electronic claims submission during the COVID19 pandemic. Second, programs in Kenya provided e-vouchers to the pregnant women via already existing mobile money system and sent reminder text messages. Third, a mobile application was developed for the program in Cameroon to offer e-vouchers and two-way communication. Fourth, a program in Tanzania empowered pregnant women with a mobile phone credit voucher so that they can communicate with healthcare providers in a timely manner.

Logic Model for Mobile Maternal Health e-Voucher Program
By synthesizing the evidence from the literature on the traditional maternal health voucher programs, as well as those involving the mobile phone system, the key elements for inputs, activities, outputs, and short-term and long-term outcomes were identified. The overview of the logic model for a mobile maternal health e-voucher program is presented in Tables 3-5. The elements that are directly related to mobile phone system appear in bold text. As the included studies suggest, the benefits package of a maternal health voucher program consists of three components: (1) maternal health services or products, (2) transportation, and (3) communication support. However, it should be noted that the logic model largely focused on maternal health services or products, given that a majority of the programs (77.8%; 21 out of 27) mainly covered maternal health services or products under the scheme. For this reason, the logic model for each of the three components was developed and presented in Tables 3-5, but the specific descriptions below focus only on  the maternal health services and products in Table 3. Various elements were identified for multiple times in different studies, but presented without duplication in the logic model framework.

Inputs
The elements for inputs were grouped and organized into the following five categories: (1) infrastructure and system, (2) organization, (3) staffing, (4) funding and resources, and (5) tools.
First, both traditional and mobile e-voucher programs require basic infrastructure and system such as health information management system or electricity. For mobile e-voucher programs, mobile money platforms or mobile applications were needed as additional inputs.
Second, examples of organization elements included the voucher management agency (VMA), contracted facilities, and independent audit or evaluation agencies.
Third, key elements for the staffing were voucher distributors, community health workers, volunteers, study coordinators, and project managers. In the case of mobile evoucher programs, community health workers or voucher distributors should be provided with support for mobile communication.
Fourth, in terms of funding and resources, overall funding for the program and the financial resources for various incentive mechanisms were identified. The examples of such mechanisms included conditional cash transfer for mothers with four ANC visits, financial support for volunteers' travel, a small premium to compensate facilities for administrative burden, and incentives for community health workers for identifying beneficiaries.
Fifth, as for the tools, the voucher, basic supplies and equipment, poverty grading tool for means testing, treatment guidelines, and facility assessment tool for contracting and accreditation were identified. Additionally, a theoretical framework for program design was suggested as a key tool.
First, preparatory activities were required for the program design. The examples included formative research, birth rate estimation, and revenue planning. In addition, benefits package design, selection of the target and eligibility criteria, review on current standard MCH services, field visits, selection of providers, development, and needs assessment of the communication strategy were performed in the previous voucher programs. Another important element was the design of a health information management system, which can involve electronic systems.
Second, activities related to sensitization were identified as follows. The methods for sensitization were diverse, such as radio, drama, posters, events, or multimedia campaigns. The targets for the sensitization were community leaders, school, church, traditional healers, or political leaders. Additionally, the mobilization of local NGOs and community health workers, and home visits for voucher promotion, could be considered.
Third, training, workshop, and mentorship were key elements for the most voucher programs. The targets for the training included health workers, midwives, community health workers, and so on. Monthly group meetings, mentorship, or training via mobile applications were also suggested in the published literature.         Fourth, payment was one important process for a voucher program. The activities for the payment involved reimbursement, verification of vouchers, claims processing, fraud control, adjustment of salaries or reimbursement rates, and cost minimization evaluations. Optional elements included transportation subsidy payments made by health centers and accommodation of installment payment.
Fifth, activities for implementation included those required in the early stage and those that happen at a later stage. The activities for the early-stage implementation were preparation of facilities, dissemination of project information, engagement of traditional birth attendants, pilot test, accreditation, and contracting. Once these activities were completed, the identification, screening and enrollment of the participants, distribution or sale of vouchers, quality assurance, communication with providers, comorbidity assessment, stratification and/or randomization, and project management should be carried out. In addition, health education and promotion activities, operation of one stop shops, scale up planning and implementation, and cluster randomized controlled trials can be considered.
Sixth, activities related to monitoring and evaluation involved supervision, audits, surveillance, and registry reviews. For a follow-up with clients, various activities were performed such as pre-and post-surveys, client satisfaction surveys, field testing of data collection forms, data collection and geo-mapping via mobile phone, validation of claims, verbal autopsy on maternal deaths, and survey by an independent market research agency. In addition, health technology assessment, cost-effectiveness analysis, evaluation of midwives, and delisting of facilities were conducted in some voucher programs.
First, outputs related to tools included manuals, reports, guidelines, patient-held record, reporting templates, and anti-fraud policies. Additionally, the number of supplies or equipment provided or the average number of equipment were considered as outputs for the tools.
Second, outputs for the staffing were the number of staffs, newly hired providers, and staffs trained, or the percentage of facilities with at least one doctor. In addition, the average number of human resources for the participating facilities were suggested.
Third, outputs related to payment were provider payment rates, time from invoice to payment, midwives' monthly income, and unofficial payments regardless of voucher use.
Fourth, the number of meetings, marketing activities, or trainings and timeliness of planned activities were considered as outputs related to implementation. Additionally, the number of pregnant women who received sensitization campaigns or who were able to activate GIS feature and feedback into the mobile platform were considered in a mobile maternal health program.
Fifth, outputs for the facilities were identified in the context of accreditation and quality. Examples included the number of accredited facilities, facilities that were improved, or districts covered under the voucher program. Those related to quality improvement were the percentage of facilities with an adequate infrastructure, facilities offering 24/7 services, or facilities with quality improvement activities.
Sixth, outputs related to voucher use included the number of vouchers distributed, sold, used, refunded, fraud cases, or voucher claims submitted. In addition, the percentage of participants who completed follow-up or who do not follow procedures as allocated were considered for the experimental studies. Other examples included the percentage of contamination; percentage of eligible women who received voucher; average length of follow-up; the number of women identified, contacted, and registered; the number of visits to the villages; and increased demand or coverage.
First, the service utilization outcomes related to delivery were frequently mentioned in the included studies. Examples of such outcomes were the proportion of estimated deliveries by the voucher, percentage or number of institutional deliveries, number of deliveries by skilled birth attendants, and percentage of home delivery.
Second, the service utilization outcome for the ANC, PNC, and other services involved the proportion of women with at least four, three, or two ANC visits; PNC visits; the number of each service delivered by the voucher; or increased equity in service utilization. In addition, gestational age at first ANC visit, the number of women who completed the sequence of services, and the percentage of women who sought care within 1 h from the onset of symptom were considered.
Third, the outcomes related to staff and resources investigated the job satisfaction in terms of workload, salary and staffing, challenges in identifying beneficiaries or distributing vouchers, effects on workload, and providers' attitudes toward the program. In addition, motivation of providers for higher quality service and changes in the resource use were included.
Fourth, women's experiences were considered as one of the important short-term outcomes. These outcomes were aimed to recommend the voucher to a friend, awareness of the program, the facilitator, barriers or reasons for voucher use, and challenges faced by the women. Additionally, women's decisions on having more children, proportion of births prepared, perceived barriers, knowledge, and attitude and practice towards the MCH services were discussed.
Fifth, the quality outcomes addressed facility level issues such as percentage of facilities that adequately considered medical history, created rapport, and prepared for equipment and supplies. In addition, service waiting hours, round-the-clock service availability, increased efficiency in service delivery, and knowledge and skills of providers were suggested. The quality outcomes were also explored from the clients' perspectives, such as satisfaction with overall experience, respect shown by the staff, perceived quality, perception on contracted facilities and quality of care, or privacy issues.
Sixth, the outcomes related to costs included out-of-pocket expenditure, average total cost, reimbursement costs or weighted average costs for each service, and incremental cost per institutional delivery. Other options were explored in the included studies, such as percentage with additional emergency cost, costs of setup and intervention, protection from financial catastrophe, or net cost per family. Key elements for cost-effectiveness studies were also examined, such as direct medical costs, direct non-medical costs, indirect costs, total cost of base wages, or total direct financial assistance to beneficiaries.
Lastly, competition and governance outcomes were explored in the included studies, given that the voucher programs in principle are intended to encourage competition among providers and better governance by empowering the clients [116]. The examples included competition among providers, increased client choices, bypassing of low-quality service, checks and balance mechanisms, accountability, and opportunities for learning and adapting to local settings.

Long-Term Outcomes
Long-term outcomes identified from the studies included (1) maternal and neonatal health outcomes, (2) cost-effectiveness, (3) sustainability of the program, and (4) integration into the health system. Among these, maternal and neonatal health outcomes were most extensively discussed in the included studies. For example, neonatal mortality, infant mortality, maternal mortality, institutional perinatal mortality, and still birth rates were key elements for long-term outcomes. In addition, outcomes related to complications such as post-partum hemorrhage, obstructed labor, or birth asphyxia were examined. Furthermore, maternal and neonatal health outcomes for cost-effectiveness evaluations were explored, such as deaths averted, DALYs averted, or life years saved per 1000 vouchers distributed. In addition to these maternal health outcomes, the cost-effectiveness outcomes were investigated, such as cost per death averted or life year saved, and cost per DALY averted. Other long-term outcomes included sustainability of the program and integration into the existing health system.

Cross-Validation of the Logic Model
For cross-validation purposes, the logic model developed from this interpretive synthesis of evidence was compared with a logic model published in a previous study on the mobile maternal health e-voucher program in Cameroon [29]. All the elements identified from their study were already included in the logic model developed from this systematic review. Figure 2 shows the overview of the logic model from the Cameroon study and the results of cross-validation. Their study did not include long-term outcomes due to the relatively short (18-month) study period. Most of the elements were shared across the traditional paper-based voucher programs and mobile e-voucher programs. Other elements such as feedback into the mobile platform, activation of the GIS feature, and reminder text messages were applicable only for mobile e-voucher programs. In addition, elements that were transitioned to mobile platforms were identified. Examples of such elements included the distribution of e-vouchers and mobile phones, instead of paper-based vouchers, follow-up contact, and data collection via mobile applications.

Discussion
By conducting an interpretive review of maternal health voucher programs in LMICs, this study aimed to develop a comprehensive logic model for mobile maternal health e-voucher programs. Building upon the previous literature on both traditional voucher programs and mobile e-voucher programs, this interpretive evidence synthesis suggests a logic model that can be utilized in a real-world setting. A total of 27 maternal health voucher programs from 84 studies were thoroughly reviewed to identify key elements for inputs, activities, outputs, and short-term and long-term outcomes. Among these 27 programs, 7 programs involved mobile phone system for various purposes, including claims processing, payment for the service providers, mobile phone credit for communication between pregnant women and the providers, mobile e-vouchers, or mobile money. The elements within a logic model were categorized and organized into themes so that the final logic model is presented in a systematic way.
Although several review studies on maternal health voucher programs have been published in the last decade, none of them deal with mobile maternal health e-voucher programs. In this context, the present study contributes to the literature by discussing mobile e-voucher programs for LMICs where the average mobile subscription rates per 100 population was 104.7 in the year 2020 [117]. Additionally, mobile money has already proliferated many LMICs and changed the mechanism for financial transactions in everyday life [118]. Given this situation, the mobile e-voucher programs have already been started in a few LMICs such as Cameroon or Kenya [29,78]. By taking advantage of the most popular mode of telecommunication in LMICs, the mobile maternal health e-voucher programs can still demonstrate the proven effectiveness of the traditional voucher programs while improving their efficiency.
Despite its potential, mobile e-voucher programs may face high startup cost, such as investment in hardware, development of software systems, or additional training for the staff [23]. However, the long-term return on investment can be higher than the traditional paper voucher programs, as the costs per client will decline by saving on the administrative costs of printing and distribution, fraud control, monitoring, and claims processing and payment. For example, a previous study on a traditional maternal voucher program in Uganda suggested the use of mobile phones for making payments to reduce transaction costs [41]. As suggested in the logic model, payment is one of the important activities that can greatly improve efficiency by shifting to mobile e-vouchers.
The benefits of transitioning to mobile e-vouchers identified from the logic model can be summarized as scalability, transparency, and flexibility. First, a mobile e-voucher program has a comparative advantage in scaling up. As in the case of the Bangladesh maternal voucher program, most maternal health voucher programs indeed started with only a few districts and then scaled up to cover larger geographic areas [94]. To reach scale, keeping management costs low is essential, which can be achieved by alleviating the administrative burden [16]. In this sense, a mobile maternal e-voucher program can be a viable option because it eliminates the administrative processes of printing, stocking, and physically distributing the vouchers, which were identified as key elements in the logic model. Additionally, mobile e-voucher systems can simplify the data entry for registration and client management.
Second, transparency in voucher management can be improved in mobile e-voucher programs by electronically tracking and following up with voucher utilization and redemption. As presented in the logic model developed from this study, the follow up activities and fraud control policy were emphasized. In fact, studies argued that the fraudulent activities or fabricated voucher claim forms are the most frequently cited concerns in traditional voucher programs, so additional investment had to be made for security features on printed vouchers, such as watermarking [17,23,119]. With a mobile e-voucher, however, general program cycle from enrollment to claims processing and payment can become more transparent by allowing for real time cross-checking and tracking.
Third, mobile e-vouchers are more flexible in terms of adjusting reimbursement rates or included services. After the rollout, maternal health voucher programs may undergo adjustments in reimbursement rates to attract more providers, as discussed in a previous study in Uganda [34]. In addition, some of the services were newly added or excluded midway through the implementation, as evidenced by the voucher program in Uganda [45]. Unlike traditional paper voucher programs, the changes resulted from these adaptations can take effect immediately, with little confusion for mobile e-voucher programs. Furthermore, mobile e-voucher programs can deliver several combinations of services efficiently. As presented in the logic model developed by the present study, a maternal voucher program generally covers three key components as the benefits package-namely, maternal health services or products, transportation, and communication. The transportation component is an integral part of maternal health voucher programs in LMICs, as suggested by previous studies that reported the women's financial burden for travel costs [120]. In addition, the communication enabled by a mobile phone system can be integrated into the voucher scheme to promote behavior change communication, emergency calls, or GIS feature. All these components can be effectively managed electronically if a mobile e-voucher program is implemented.
In fact, electronic voucher schemes have been implemented in several high-income countries as well. For example, an electronic voucher program for maternal health services was launched in South Korea in 2008 [121]. The maternal e-voucher program offers approximately USD 500 worth of maternal health services for all pregnant women under the universal health insurance scheme in South Korea. A previous study reported the reduced risk of preeclampsia after the introduction of this universal voucher scheme for maternal health [122]. Another example of e-vouchers for maternal and child health in a high-income country is the Special Supplemental Nutrition Program for Women, Infants, and Children, also known as the WIC program, in the United States. A recent study showed that beneficiaries had a positive attitude towards the transition to the electronic benefits transfer (eWIC) system, which involves electronic redemption of the food items [123].
This interpretive review identified the studies on maternal health voucher programs in LMICs and synthesized evidence to develop a logic model for a mobile e-voucher program for maternal health. Although key elements for the logic model were thoroughly reviewed and presented in this study, there are still a few limitations. First, only studies published in English were included for analysis. As a result, maternal health voucher programs that did not have any publications in English could not be accessed. To minimize the risk of missing any valuable information about the programs, this study included all types of publications such as reports or issue briefs, unlike the traditional systematic review approach that generally considers only the specific study design. Second, this study could have provided more practical information if it involved voices from the stakeholders. As part of vetting process for the logic model, future research can be conducted along with stakeholder consultations. Third, a small number of studies on mobile e-voucher programs were identified and included for analysis, even though the purpose was to develop a logic model that suggests a transition from paper to mobile e-vouchers. As the uptake of the e-voucher approach is growing in other areas such as agriculture, family planning, and insecticide-treated bed nets, the application in maternal health will also likely to increase in the near future [23]. In this sense, opportunities will be available for a future systematic reviews that can rigorously assess the risk of bias of studies on mobile maternal health e-voucher programs.

Conclusions
Maternal health vouchers have been introduced in many LMICs over the past decade, and previous literature confirmed the effectiveness of such programs for increasing the utilization of maternal services for the underserved women. This interpretive review attempted to take it one step further so as to provide evidence for the recently growing mobile e-voucher programs. In doing so, key elements for the inputs, activities, outputs, and short-term and long-term outcomes were identified and systematically organized to develop a logic model. The findings from this study suggest that the majority of elements overlap between traditional paper vouchers and mobile e-vouchers. For example, longterm outcomes that should be achieved are reductions in maternal and neonatal mortality, regardless of the voucher type. However, some of the elements have the potential to be greatly improved by transitioning from paper to mobile e-vouchers. By presenting the overview of an evidence-based logic model for mobile e-voucher programs for maternal health, this study provides insight into the planning, implementation, and evaluation of the program.